The present invention relates to an ultrasonic seal. It is used in a large number of fields and particularly in the monitoring of containers containing dangerous products, precious materials, documents, etc.
The principle of monitoring by ultrasonic seals is known. It consists of fixing in a definitive manner an ultrasonic seal to a container to be monitored, said seal being provided with a unique, non-reproducible marking located in the vicinity of the preestablished seal breaking zone. This marking is obtained by structural irregularities of the matrix containing the seal, e.g. by inclusions of material or cavities distributed in a random manner.
Generally, an ultrasonic seal comprises a transducer able to convert electrical excitation into an ultrasonic wave and vice versa. The structural irregularities on the path of the transmitted ultrasonic wave give rise to ultrasonic echoes detected by the transducer, which on return supplies an electrical signal, whose complex waveform is specific to the seal. Electronic means are provided for exciting the transducer and for collecting the electrical signal supplied by it as well as for the analysis of said signal. In such a device, the ultrasonic transducer functions both as a transmitter and as a receiver, so that the seal only has a single transducer.
However, ultrasonic seals are known, which have two transducers, namely an ultrasonic transmitting transducer and an ultrasonic receiving transducer. Such a device is described in French Patent Application 2 518 751 and its structure is shown in the attached FIG. 1. As shown, the prior art seal comprises a body 1, two coaxial connections 2,3 connected to not shown electronic processing means, a transmitting ultrasonic transducer TE and a receiving ultrasonic transducer TR, both connected to the center conductors 6,7 of the coaxial connections. A conductive plate 8, forming an electric shield, is placed between the two transducers TE and TR. This plate is connected by wires 9 to external conductors 4,5 of connections 1 and 2. Each transducer is formed from a piezoelectric element 18, an electrically conductive damper or absorber 19 made from resin (e.g. epoxy resin) filled with metallic particles in a high concentration (e.g. tungsten) and a contact electrode 20. The transducers are adhered to a delay line 11 with the aid of an adhesive (e.g. cyanoacrylate).
The piezoelectric parts 18 of transducers TE and TR are adhered to the element 11 forming the delay line for the ultrasonic wave and said element can be made from Duralumin. Its ends are flush with the seal surface 12. A sealing pin or stud 13, e.g. of Duralumin is glued to the seal surface 12. It is therefore in contact with the delay line 11, said pin or stud having an acoustic identity.
As shown, the stud 13 is in three parts, namely a waveguide 14, a breaking zone 15 and an anchoring zone 16. The latter is plugged in to the cover of the container to be monitored. The stud must necessarily be broken level with the breaking zone 15 to enable the container to be opened. The parts still attached to the stud after breaking are removed by machining and the seal can be recovered, a new stud being fixed by glueing.
Although this device is satisfactory, it is limited to the case where the sealing of the container takes place by introducing a non-dismantlable stud into the actual structure of the container.
The object of the present invention is to obviate this limitation by widening the field of use of the seal. By analogy with the type of seal used by customs, the seal according to the invention is usable whenever it is appropriate to pass a safety cable around a packet or other object, or through any orifice made for this purpose in the structure of the object to be sealed.